In recent years, thanks to advances in electronics technology, mobile information terminals using a touch screen have come into wide use. In order to input a predetermined character using such a mobile information terminal, a user may press a corresponding key with a fingertip or an operation body such as a touch pen using a software keyboard, which is displayed on a display and has substantially the same arrangement as an actual keyboard. However, because the software keyboard displayed on the mobile information terminal using the known touch screen is displayed with substantially the same arrangement as the actual keyboard in a limited screen area, keyboard keys become small. This is therefore a significant strain on the user's eyes, and typographical errors are prone to occur. This problem is even more noticeable in a device with a small touch screen area.
In order to solve this problem, a method for inputting keys using a pressure sensor capable of sensing applied pressure has been proposed (for example, Patent Literature 1). In the method disclosed in Patent Literature 1, only kana characters at the tops of columns in the Japanese syllabary table (that is, “A,” “Ka,” “Sa,” “Ta,” “Na,” “Ha,” “Ma,” “Ya,” “Ra,” and “Wa,” which are top kana characters at the tops of columns each including kana characters having the same consonant among 50 kana characters in which 5 vowels are vertically arranged and 10 consonants are horizontally arranged) are allocated to blocks. When one predetermined character (for example, “A”) among the top kana characters is touched, subordinate kana characters (for example, “A,” “I,” “U,” “E,” and “O”) belonging to the touched top character are displayed. At this time, after a focus has moved to a character the user wants to select according to an amount of pressure at which the operation body has touched, the user can select the character by releasing the operation body from a contact surface. Thus, it is possible to implement a software keyboard efficiently using a limited screen area while minimizing hand and eye movements in an operation of selecting the subordinate kana characters from the top kana characters.